1. Field of the Invention
The present invention relates to an ink set suitable for use in recording color images on a recording medium by using plural color inks of at least two different colors in combination and an ink-jet recording method using such the ink set, and more particularly to an ink set, which permits the provision of bright images with sufficient image density and high quality in image recording according to an ink-jet system, and an ink-jet recording method using such the ink set.
2. Related Background Art
An ink-jet recording method is a recording method in which recording is conducted by generating and ejecting minute droplets of an ink by one of various ink ejection systems, for example, an electrostatic attraction system by application of high voltage, a system using a piezoelectric element to give an ink (colored ink) mechanical vibration or displacement, or a system in which an ink is heated to form bubbles in the ink, thereby using the pressure thus produced, and applying the ink to a recording medium such as paper to form ink dots, and is a recording method which scarcely produces noise and can conduct high-speed printing and color printing.
The images recorded by using such an ink-jet recording method as described above are applied to various purposes, for example, for the observation of surface images in the case where the recording has been conducted with plain paper or the so-called paper for ink-jet recording, or for the observation of projected images formed by projecting the recorded images on a screen or the like by an optical instrument such as a slide projector or an over-head projector (OHP), and besides, as a color separation plate upon the preparation of a positive plate for multi-color printing, a color mosaic filter (CMF) used in color display devices using liquid crystals or the like in the case where the recording has been conducted with a transparent recording medium.
On the other hand, when a color image is formed by the ink-jet recording method, a recording method by a subtractive color mixture process may be mentioned as a representative method. When the color image is formed by the subtractive color process, three primary colors of cyan (C), magenta (M) and yellow (Y) are generally used.
Colors expressible by mixing at least two of these three primary colors in the same pixel (i.e., by applying ink droplets of different colors to the same pixel so as to overlap each other, thereby conducting recording) are basically the following four colors. Therefore, colors expressible by the three colors of C, M and Y are basically 7 colors of C, M, Y, red (R), green (G), blue (B) and black (Bk). A color image made up of these seven colors is designated false color image.
(1) C+M.fwdarw.B PA1 (2) C+Y.fwdarw.G PA1 (3) M+Y.fwdarw.R PA1 (4) C+M+Y.fwdarw.Bk.
Namely, when recording is conducted by such a recording method, it is basically only necessary to provide 3 inks of different colors for three primary colors of C, M and Y.
When Bk is expressed by the three primary colors of C, M and Y, however, it is printed by ejecting ink droplets of the three primary colors, C, M and Y so as to overlap one another, and the inks of the respective colors are used in substantially the same volumes for the ink droplets. Therefore, the Bk dot formed by applying the three ink droplets to a recording medium so as to overlap one another is more enlarged than it needs compared with dots of portions recorded in colors other than Bk, and dot gain occurs at such a portion, so that there are caused problems that unnatural image are formed and that a failure in ink absorption in the recording medium occurs because of increased ink quantity at the portion. Further, since frequency of recording by Bk is generally high upon recording, these problems are important.
For this reason, besides the inks of the three primary colors, C, M and Y, a Bk ink is added for the expression of Bk, whereby the four inks of different colors are generally used. However, when recording is conducted with the four inks of C, M, Y and Bk colors, droplets of two inks of different colors are ejected so as to overlap each other at portions printed in B, G and R colors. At this time, it takes some time to absorb ink droplets increased in quantity, in particular, when the recording medium is wood free paper containing a size. Therefore, the ink droplets enter interfiber spaces of the paper in all directions of the recording medium, including the sectional direction thereof, during the absorption. As a result, uneven dots are formed, so that irregular dot gain, or the so-called feathering occurs, which causes a problem that edges of a recorded image are blunted, resulting in a blurred image.
Besides, when two ink droplets of different colors are ejected adjoiningly to each other, the ink droplets partially mix with each other at the boundary between these colors before they are fixed. As a result, bleeding occurs between the different colors, which also causes a problem that a blurred image is formed.
As a method for solving the problems as described above, it is considered to make the rate of penetration of ink droplets into a recording medium high. By doing so, the surface tensions of individual inks are reduced, so that it can be realized to improve the wettability of the inks to the recording medium. However, recording with the inks reduced in surface tension may cause a problem that dots are more enlarged than they need on a recording medium, resulting in dot gain, or that since the rate of penetration is made high, coloring materials contained in the inks also markedly penetrate into the interior of the recording medium, resulting in reduction of color density.
In particular, although a Bk ink is often used also in recording of picture images, its frequency of use becomes higher when recording characters. Therefore, when the color density of the Bk ink at the time it is used in the recording of characters is made higher than those of other colors, and sharp edges free of any unnatural dot gain are formed, an image, which leaves a more favorable impression to look at it, can be provided. Thus, for example, when the surface tension of only a Bk ink among many color inks is raised to some extent, and recording is conducted with such an ink to lower its rate of penetration into a recording medium, a bright black image with high color density can be surely obtained. However, such an attempt results in a failure to solve the problem of feathering caused by applying the Bk ink and another ink so as to overlap each other, and the problem of bleeding occurred at boundaries between the Bk ink and the other inks.